Bass leaves after amp warms up?


I don't understand-after my Musical Fidelity M6i amp warms up for about an hour I notice the deep bass & kick drum aren't the same.
They sound less musical with loss of weight/depth.The notes are there but the moving of air have left.Sound is has much less impact and boreing.
I had the same problem with Bryston amp so there is no defect with amps nor with the rest of my equipment/
PSB Synchrony one speakers,AQ cables,Bryston CD Player.
My question has anyone heard similar & is there a plausable reason?
fishing716
For the record I have a steady girlfriend that hears when the music changes..
Just the other day said "what happened" like the air out of a balloon.
It's been a while since I used stock power chords but will give it a shot.
Did you ever try a different power cord on the amp?
Lets get back to the electrician visit....

When the electrician visited, did he monitor the power before, during and after you had these issues? If it included during your problems (what would be the point otherwise?), did he not hear the issues too?
Has anybody yet suggested cold solder joints somewhere in the speaker or DC saturated inductors.